Stereo-quadraphonic matrix system with matrix or discrete sound reproduction capability

ABSTRACT

A sound system for producing at least four discrete sound outputs from respectively different directions from a listener wherein the audio and directional information representing the sound outputs are recorded on a stereophonic disc record, or similar record medium having a two-track capability. The system includes recording apparatus having two matrix encoders for separately and differently matrixing four input signals to produce two pairs of composite signals each containing three of the input signals with preselected amplitude and phase relationships, at least one of the signals having a different phase in each of the composite signals of a pair. The amplitude and phase relationships are such that four combinations of the two composite signals of each pair will yield four separate signals in which respective ones of the four input signals are predominant. The two composite signals produced by one of the encoders are recorded at baseband frequency on respective walls of a stereophonic disc record, and the two composite signals from the other encoder, which have different phase relationships than the corresponding composite signals of the other pair, are utilized to amplitude modulate respective carrier signals which are also recorded on the walls of the record groove. Apparatus for reproducing the record includes filters for separating the baseband frequencies from the carrier frequencies, detectors for recovering the modulation from the carriers, and a pair of decoders, one for decoding the baseband frequency signals and the other for decoding the two composite signals recovered from the carriers. Each decoder delivers four output signals in which respective ones of the four input signals are predominant and each containing unwanted signal components. Corresponding signals from the two encoders are combined, and by virtue of differences in the initial encoding, the desired predominant signals are enhanced and the unwanted signal components are canceled, thereby to produce four discrete output signals, corresponding in all respects to the input signals, for application to respective loudspeakers suitably positioned in a listening area.

United States Patent [191 Bauer 51 Sept. 25, 1973 1 STEREO-QUADRAPHONICMATRIX SYSTEM WITH MATRIX OR DISCRETE SOUND REPRODUCTION CAPABILITY [75]Inventor: Benjamin B. Bauer, Stamford, Conn.

[73] Assignee: Columbia Broadcasting System, Inc.,

New York, NY.

[22] Filed: Apr. 13, 1972 [21] Appl. No.: 243,800

[52] US. Cl 179/1 GQ, l79/100.4 ST

[51] Int. Cl H04r 5/00 [58] Field of Search 179/1 GO, 1 G, BT,

179/100.4 ST, 100.1 TD

[56] References Cited UNITED STATES PATENTS 3,686,471 8/1972 Takahashi179/1 GQ 2,874,221 2/1959 Dauguet 179/15 BT 3,067,292 12/1962 Minter179/100.4 ST

3,085,203 4/1963 Logan et 211.... 325/ 3,401,237 9/1968 Takayanagt...179/100.4 ST

OTHER PUBLICATIONS Why the 4-Channel War Need Not Take Place by Feldman,Audio Magazine, July 1972.

Primary Examiner-Kathleen H. Claffy Assistant Examiner-Thomas DAmicoAtt0rneySpencer E. Olson [5 7] ABSTRACT A sound system for producing atleast four discrete sound outputs from respectively different directionsfrom a listener wherein the audio and directional informationrepresenting the sound outputs are recorded on a stereophonic discrecord, or similar record medium having a two-track capability. Thesystem includes recording apparatus having two matrix encoders forseparately and differently matrixing four input signals to produce twopairs of composite signals each containing three of the input signalswith preselected amplitude and phase relationships, at least one of thesignals having a different phase in each of the composite signals of apair. The amplitude and phase relationships are such that fourcombinations of the two composite signals of each pair will yield fourseparate signals in which respective ones of the four input signals arepredominant. The two composite signals produced by one of the encodersare recorded at baseband frequency on respective walls of a stereophonicdisc record, and the two composite signals from the other encoder, whichhave different phase relationships than the correspond ing compositesignals of the other pair, are utilized to amplitude modulate respectivecarrier signals which are also recorded on the walls of the recordgroove.

Apparatus for reproducing the record includes filters for separating thebaseband frequencies from the carrier frequencies, detectors forrecovering the modulation from the carriers, and a pair of decoders, onefor decoding the baseband frequency signals and the other for decodingthe two composite signals recovered from the carriers. Each decoderdelivers four output signals in which respective ones of the four inputsignals are predominant and each containing unwanted signal components.Corresponding signals from the two encoders are combined, and by virtueof differences in the initial encoding, the desired predominant signalsare enhanced and the unwanted signal components are canceled, thereby toproduce four discrete output signals, corresponding in all respects tothe input signals, for application to respective loudspeakers suitablypositioned in a listening area.

19 Claims, 8 Drawing Figures /0 C M T/ l 4- l DELAY Lb Ell/CODE? f/ R 2HG r r/ /52 FILTERS /60 0} 05m)" 2 A94 //86 DELAY 0 LA) /70 /74 fE/VCOOER /78 M00 2 050.63% )F/LT'RS H62 358 /64 T1 2-- M00 -,l'f

/72 I 72 ms n92 STEREO-QUADRAPI'IONIC MATRIX SYSTEM WITH MATRIX ORDISCRETE SOUND REPRODUCTION CAPABILITY BACKGROUND OF THE INVENTION Thisinvention relates to audio systems, and in particular to a sound systemadapted to record four-or more individual channels of audio informationcontaining directional information on a two-track record medium and toreproduce the recorded information as four discrete audio output signalshaving the directionality of the original input signals.

In commercial stereophonic systems, two independent signals respectivelymodulate the two channels (left and right walls) of a single grooverecord in two perpendicular directions. Thus, the groove variations inone channel represent one function of the two signals and those in theother channel represent another function of the two signals. Typically,the groove is cut with modulation in each wall of the grooverepresenting each signal and with lateral modulation representing thesum of the signals and vertical modulation representing the differencebetween the signals.

Since the commercialization of stereophonic sound reproduction, effortshave been made to record three or more signals on a two-channel recordfor the obvious reason that the sound distribution of the reproducedinformation will improve as the number of signal sources increases. Forexample, three-channel disc recording systems have been proposed inwhich a selected fraction of a third signal corresponding to the sounddetected by a centrally disposed microphone is added to the two signalscorresponding to the sound detected by spaced left and right microphonesto record a dependent sum, or center, channel on the stereophonicrecord. Alternatively, it has been proposed to subtract a selectedfraction of the third signal from one, and add it to the other of saidtwo signals and to thus record a dependent difference channel on thestereophonic record. However, such a difference channel is notcompatible with monophonic players because the vertical groovemodulation cancels for monophonic reproduction, nor with stereophonicplayers, which produce an unnatural sound during reproduction.

In co-pending application Ser. No. 169,219 filed Aug. 5, 1971 by thepresent applicant and others, and assigned to the assignee of thepresent application, a system is described for recording and reproducingfour independent, discrete sound signals designated L,, R,, L and R, ona two-channel medium, such as a stereophonic disc phonograph record. Inaccordance with the therein described invention, modulated carriersignals are utilized to provide, in effect, two additional recordingchannels so as to permit recording of four signals independently on theotherwise two-channel medium. Briefly, two baseband channels in thefrequency domain from about 20 to 15,000 Hz are respectively recorded onthe left and right channels of the disc, and two single sidebandamplitude modulated carrier signals spanning the frequency range'between about 20,000 and 35,000 Hz are also recorded in the left andright channels, respectively. When the record is reproduced with aphongraph pickup sensitive to frequencies up to about 35,000 Hz, thebaseband frequencies separated from the modulated carriers by means offilters, and the modulation on the carriers detected, four separateindependent information-carrying channels are obtained, each having acapacity of approximately 15,000 Hz. These four channels are used torespectively record one of the aforementioned signals L,, R,, L and R ina completely discrete and independent manner.

This result is achieved by matrixing (i.e., adding with linearcoefficients) the four audio signals to form four matrixed signals, twoof which when recorded at baseband will provide a reasonably pleasingsound display when reproduced on a conventional stereophonic phonograph,and the other two of which when employed to amplitude modulate carriersignals also recorded on the separate channels of the stereophonicrecord, will upon replay and detection provide signals which whencombined with the signals derived from the baseband channels producefour output sound signals that are completely discrete and independentof each other.

More particularly, in this prior system the four signals are matrixed orcombined as follows: the left baseband channel signal, for conveniencedesignated L equals (L;+ L,,); the right baseband channel signal Requals (R, R,,); the left carrier channel, designated L equals (L, L,,);and the right carrier channel, designated R equals (R, R,,). The sumsignals (i.e., L and R are recorded as the basebands of the two channelsand the carrier signals amplitude modulated by the difference signals(i.e., L and R are recorded as the high-frequency signals on thecorresponding channels of a two-channel stereophonic record. Uponreproduction of the disc with a pickup having a sensitivity up to theupper frequency of the modulated carrier, and demodulation of thecarrier signals, the reproduced signals are matrixed or combined asindica'ted below to obtain the four original channels: L]: L11 Ln R]:R11+ Rn b 11 'rz b Tl T2 While the record prepared by the describedtechnique provides discrete output signals, it is not compatible" in thesense that it can not be reproduced satisfactorily on a conventionalsterophonic phonograph because its pickup usually is unable to reproducefrequencies above about 20,000 Hz; thus, only the baseband channelswould be reproduced and those recorded in the carrier mode would belost. Moreover, it has been determined by listening tests as well as bytheoretical considerations that reproduction of the baseband channels isnot entirely satisfactory because upon stereophonic replay the pair ofsignals L, and L,, appears to be localized in one loudspeaker and theother pair of signals R, and R, appears to be localized at the other,instead of filling the space between them; thus, space perspective isnot satisfactorily portrayed.

SUMMARY OF THE INVENTION It is the principal object of the presentinvention to provide improved apparatus for recording four discreteaudio information signals on a two-channel disc record and apparatus forreproducing the same to deliver four discrete output signals, whichrecord is also compatible" with existing sound systems in the sense thatit can be acceptably reproduced on a conventional stereophonicphonograph, as well as on certain existing quadraphonic reproducingapparatus of the matrixed type to deliver four output sound signals,albeit not discrete signals, for presentation on respectiveloud-speakers.

This and other objects of the invention are attained by utilizing atechnique similar to that described in the aforementioned application inthe respect that carrier signals are used to obtain two additionalrecording channels, but differing therefrom in the manner in which thefour discrete input signals are 'matrixed prior to recording. Instead ofthe simple sum-and-difference matrixing used in the prior system, fourdiscrete inputsignals, for convenience designated L,, R,, L,, and R,,,are matrixed in two different matrix encoders each adapted to producetwo composite signals each containing at least three of the inputsignals with preselected amplitude and phase relationships, at least oneof the signals having a different phase in each of the compositesignals. The amplitude and phase relationships are such that fourcombinations of the two composite signals will yield four separatesignals in which respective ones of the four input signals arepredominant. The two composite signals produced by one of the encodersare recorded at baseband on respective walls of the record groove, andthe two composite signals from the other encoder, which have thejustdescribed properties but have phase-relationships between thesignals contained therein differing from those in the first pair ofcomposite signals, are utilized to amplitude modulate respective carriersignals which are also recorded on the two walls of the record groove.Designating the two composite signals from one encoder L and Rrespectively, and those from the other encoder L and R respectively, theL signal from one encoder is recorded at baseband and a carriermodulated with the L signal from the other encoder are recorded on theleft groove wall, and the R signal from the said one encoder is recordedat baseband and a carrier modulated with the R signal from the otherencoder are both recorded on the right groove wall.

The apparatus for replaying the record includes a stereophonic pickupcapable of reproducing the highest of the modulated carrier frequencies,suitable filters for separating the baseband frequencies from thecarrier frequencies, means for detecting the modulation on the carriers,and a pair of matrix decoders, one for dematrixing the two encodedbaseband frequency signals and the other, having different connections,for decoding the two encoded signals recovered from the carriers. Eachof the decoders is operative to deliver four output signals in whichrespective ones of each of the four input signals are predominant andaccompanied by two others of the input signals but at lower amplitude.Corresponding output signals from the two decoders are then combined inrespective summing networks, and by virtue of the differences in themanner in which the two encoders initially matrixed the four inputsignals, and the two decoders have decoded the matrixed signals, thedesired predominant signals are enhanced and the lower amplitude signalsare canceled, thereby to produce four discrete output signals,corresponding in every way to the input signals, for application to fourloudspeakers positioned at the left front, right front, left back andright back corners of a listening area.

In addition to its discrete channel performance when replayed on thejust-described reproducing apparatus, the record produced in accordancewith the invention is compatible" in the sense that it may be replayedon a conventional stereophonic phonograph having a pickup sensitive overonly the baseband frequency range with results superior than is obtainedfrom a conventional stereophonic record because of the presence ofbacksignal information in addition to the left and right front signals inthe two composite signals recorded at baseband, and in that its basebandchannels can be reproduced on existing matrix-type quadraphonic playbacksystems to provide a high degree of four channel realism.

Thereproducing apparatus of the invention is also compatible withexisting quadraphonic disc records having recorded thereon compositesignals matrixed in a manner compatible with the baseband decoder; inthis situation, the second decoder for the composite signals derivedfrom the carriers would not be operative.

Furthermore, while the invention is summarized above and will bedescribed herein in connection with a disc record, it is equallyapplicable to other recording media and/or broadcasting or transmissionsystems normally having two channels but can, by carrier techniques, beconverted to have four channels. In a more general sense then, theinvention is useful for recording or transmitting multi-channelinformation over a multichannel system, with good reproduction, usingbut two of the channels thereof.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects, features and advantagesof the invention will be evident, and a better understanding of itsconstruction and operation will be had from the follow ing descriptionand accompanying drawings, in which:

FIGS. 1 and 2 are block diagrams illustrating preferred embodiments oftwo encoding matrixes for separately matrixing four discrete channels ofaudio information;

FIGS. 3 and 4 are block diagrams illustrating preferred embodiments ofmatrix decoders for deriving individual signal components from compositesignals matrixed by the encoders of FIGS. 1 and 2, respectively;

FIG. 5 is a block diagram illustrating a preferred embodiment of arecording system arranged according to the invention for recording fourchannels of audio in formation on a two-channel disc record;

FIG. SA is a block diagram illustrating in more detail a portion of thesystem of FIG. 5;

FIG. 6 is a diagram illustrating the frequency distribution of thebaseband and single sideband carrier signals recorded on one channel ofa two-channel stereophonic record; and

FIG. 7 is a block diagram illustrating a preferred embodiment of areproducing system arranged according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT four channels, the input signalsL;, R,, L and R, are

Upon replay and demodulation the signals L R L and R become availablefor recombination to recover the original signals. It is well known thatin every case where equations of this form are linearly independent,they can be solved to retrieve the four original signals in thefollowing form:

L? 11 41 iz ri l3 7'2 14 T2 It will be evident that in the matrixingtechnique utilized in the co-pending application, the a" coefficients inequations 1 through 4 are as follows: Eq. (1) 1, l, 0, Eq. (2) 1,l, O,0; Eq. (3) 0, 0, I, 1; and Eq. (4) 0, 0, l,l. Solution ofequations l (2)and (4) reveals that the b" coefficients in Eq. are :z r0,0;in Eq. (6)k. %0, 0; Eq. (7)0, 0, A95; and Eq. (8) O, O, re-b. Substitutingequations (1) to (4) into equations (5) to (8) using these coefficientsproduces the four original signals L,, R,, L, and R The principlerepresented by this analysis is extended and utilized in the presentinvention.

In a preferred embodiment of a four-channel recording system arrangedaccording to the present invention, as shown in FIG. 5, a pair of matrixcircuits l0 and 12 each receive four audio signals from a suitablefourchannel source, such as a master tape (not shown) or four individualmicrophones, via input terminals l4, l6, l8 and 20, respectively. Theinput signals are designated left front (L,), left back (L right front(R,) and right back (R,,) and indicate the location in a listening areain which they are to be reproduced.

The matrix circuits and 12, illustrated in FIGS. 1 and 2, respectively,are very similar in construction and operation and embody the inventiondescribed in appli cants co-pending application Ser. No. 124,135, filedMar. 15, 1971 and assigned to the assignee of the present application.The encoder of FIG. 1 has four input terminals 14', 16, 18' and 20'(they correspond to the input terminals of the recording system of FIG.5) to which the four signals L,, L R and R,, depicted as inphase signals(when of the same frequency) and of equal amplitude, are respectivelyapplied. With the switches S and S in the positions indicated, the fullL, signal is added in a summingjunction 22 to 0.707 of the R signal, theoutput of the summing junction being applied to a phase-shifting network24 which introduces a reference phase-shift 111 which variescontinuously with frequency. The full R, signal at terminal 20' is addedin a second summing network 26 to 0.707 of the L, signal appearing atinput terminal 16, and the output is passed through a second ill-network28 which also provides the reference phase-shift 41. The L,, and R,signals are also applied to respective ill-networks 30 and 32, each ofwhich provides a phase ahift of III-90 and wherein the all-functions areessentially the same. The

7 full signal appearing at the output of network 24 is output terminal36 a composite signal designated L Similarly, the full signal fromnetwork 28 is added in summing junction 38 to O.707 of the signal fromnetwork 32 to produce at its output terminal 40 a composite signaldesignated R The composite signals L and R are convenientlycharacterized by the phasor groups 42 and 44, respectively, and may berecorded on a two-track tape recorder in the form of encoded signals fora subsequent'recording on a disc record, or they may be encoded directlyon a disc record using a conventional stereophonic record cutter.

' It will be observed from examination of phasor groups 42 and 44 thatencoder 10 positions the full L,

and R, signals in phase with each other, causes the R components in thetwo composite signals to be substantially in quadrature with each other,and the L,, components in the two composite signals to also besubstantially in quadrature with each other. Additionally, the L, and R,signals at output terminal 36 lead and lag, respectively, the L and R,signals at output terminal 40, and the R signal component is alignedwith and in phase with the L, signal at terminal 36 and the L, signalcomponent is aligned with but is out-ofphase with the R, signal inphasor group 44.

By slight modification of the encoder of FIG. 1, illustrated in FIG. 2,the four input signals can be encoded in a different manner whereby thephasors representing its composite output signals are the complements ofthe corresponding phasors representing the composite signals deliveredby the FIG. 1 encoder. More specifically, the full L, signal is added ina summing junction 46 to -0.707 of the R signal, the output of thesumming junction being applied to a phase-shifting network 48 whichintroduces the reference phase-shift 41, which has the same function asin the encoder of FIG. 1. The full R, signal at terminal 20" is added insumming network 50 to 0.707 of the L,, signal at input terminal 16" andthe output is passed through the ill-network 52, which also provides thereference phase-shift 111. The L, and R, signals are also applied torespective Ii-networks 54 and 56, each of which provides a phase-shiftof ll-. The full signal appearing at the output of network 48 is addedin a summing network 58 to 0.707 of the signal appearing at the outputof network S4to produce at its output terminal 60 a composite signaldesignated L Similarly, the full signal from network 52 is added insumming junction 62 to 0.707 of the signal from network 56, to produceat its output terminal 64 a composite signal designated R As with theencoder of FIG. 1, in the phasor groups 66 and 68 representing thecomposite signals L and R respectively, the L, and R, signals are inphase and each have unity value, the L and R signal components in one ofthe composite signals are in substantial phase quadrature with thecorresponding signal components in the other composite signal, the L andR, signals at output terminal 60 lag and lead, respectively, the L, andR,, signals at terminal 64, the R signal in phasor group 66 is alignedwith but out-of-phase with the L, signal, and in phasor group 68 the L,signal is aligned with and in phase with the R, signal.

As will be seen from the description to follow, it is the differences inthe relative positions of the phasors representing the composite leftand right" signals delivered by the two different encoders, and therecognition that by decoding of the composite signals produced by eachand appropriate combination of the decoded signals, it is possible toderive the signals L,, L,, R, and R, as discrete, completely independentsignals. Although such decoding and combining occurs in the reproducingapparatus it will be ,helpful to an understanding of the invention tohere describe the decoding technique, before continuing with thedescription of the recording system of FIG. 5.

Essential to achieving the objects of the invention was applicantsrecognition that by utilizing appropriate matrix decoders of the typedescribed in his aforementioned co-pending application Ser. No. 124,135to decode the signals produced by the encoders of FIGS. 1 and 2 it ispossible to obtain two sets of four decoded signals wherein the primarysignals L L R and R, are identical in both sets while theircorresponding accompanying signals are equal and out-of-phase wherebywhen corresponding decoded signals of the two sets are added the desiredsignals are augmented and the accompanying signals are canceled tothereby obtain a set of four output signals corresponding in every wayto the original signals supplied to the encoders. To this end, thedecoder illustrated in FIG. 3 is used to decode the composite signals Land R produced by encoder l0, and the decoder illustrated in FIG. 4,which closely resembles the circuit of FIG. 3, is used to decode thecomposite signals L and R produced by encoder 12. The decoder of FIG. 3,which is also described in applicants co-pending application Ser. No.118,272 filed Feb. 24, I971, includes a pair of input terminals 70 and72 to which the composite signals L and R are respectively applied. Thesignal applied to terminal 70 is applied in parallel to andphase-shifted by a pair of IIJ-IIEIWOI'ICS 74 and 76, and thecomposite Rsignal applied to input terminal 72 is applied in parallel toiii-networks 78 and 80. These ill-networks are of the type previouslydescribed, the networks 74 and 80 introducing a reference phase-shift ofill and the networks 76 and 78 introducing a phase-shift of Ill-90 1 Theoutput signals from networks 74 and 80 are respectively applied directlyto the left front" output terminal 82 and to the right front outputterminal 84. Equal portions of the outputs of networks 74.and 78 aresummed in a summing junction 86, the output of which is applied to theright back output terminal 88, and equal portions of the outputs ofnetworks 76 and 80 are inverted and added in a second summing network90, the output of which is applied to the left back output terminal 92.As a result of the described phase-shifting and summing action fourdecoded signals designated L' L' R,, and R',, appear at output termainls82, 92, 88 and 84, respectively, and have the composition depicted byphasor diagrams 94, 96, 98 and 100, respectively. It is seen that thepredominant signals in the four decoded signals, namely, L,, L,,, R, andR,, have the same relative amplitude and phase as the correspondingsignals applied to the encoder of FIG. 1, but that the predominant frontsignals are accompanied by reduced amplitude signals from the back pairof channels and the predominant back" signals are accompanied by reducedamplitude signals from the front pair of channels.

The decoder of FIG. 4 likewise has a pair of input terminals 102 and 104to which the composite signals L and R initially encoded by endoder 12are respectively applied. The L signal is applied in parallel to a pairof phase-shifting networks 106 and 108 which respectively introduce areference phase shift 11; and a phase shift of ill-, and the R signal isapplied in parallel to a pair of similar phase-shift networks and 112.As in the decoder of FIG. 3, the outputs of phaseshift networks 106 and112 are respectively applied directly to the left front output terminal114 and the right front output terminal 116. Equal portions of theoutputs of networks 108 and 112 are summed in a summing junction 118,the output of which is applied to the left back output terminal 120, andequal portions of the outputs of networks 106 and 110 are inverted andsummed in a second summing network 122, the output of which is aplied tothe right back output terminal 124. Again, the four decoded signalsappearing at the output terminals, represented by phasor groups 126,128, and 132, respectively contain predominant signal components L';,L',, R, and R,, each accompanied by reduced amplitude signals from twoother channels. Again, also, the predominant signals have the samerelative amplitude and phase relationship that they exhibited at theinput terminals of the encoder of FIG. 2. 7

Inspection of the two sets of phasor groups from the two encodersreveals that the predominant signals L,, L,, R and R, in both areidentical in relative phase and amplitude to the corresponding signalsat the input terminals of their respective encoders, but that theircorresponding accompanying signals are equal and out-ofphase. Forexample, comparison of phasor groups 94 and 126 shows that the L; signalin both is equal and in phase, whereas the signals 0.70711, and 0.707L'in one are equal and out-of-phase with corresponding components in theother. Therefore, when the signal at terminal 82 of the FIG. 3 decoderis added to the signal at terminal 114 of the decoder of FIG. 4, thedesired L, signal will be augmented (doubled) and the undesiredaccompanying signals 0.707L' and 0.707R', will be canceled. The same istrue of the composite signals appearing at output terminals 92, 88 and84 of the decoder of FIG. 3 and the corresponding signals at the outputterminals 120, 124 and 116 of the decoder of FIG. 4. Thus, by matrixingthe four orignal input signals with the differing encoders of FIGS. 1and 2, and decoding the composite signals produced thereby with thediffering decoders in FIGS. 3 and 4, respectively, and by combining theoutputs of the two decoders in an additive mode, a set of four outputsignals is obtained which in every way correspond to the orignal signalssupplied to the encoders; that is, signals which are completelyindependent of each other and contain the orignal directionalinformation.

While two encoder circuits have been separately illustrated in FIGS. 1and 2, and two decoder circuits have been shown in FIGS. 3 and 4, tographically show how the signals are matrixed and dematrixed to attainthe objects of the invention, inspection of these figures will show thatthe encoder of FIG. 2 is the morror image of the encoder of FIG. 1 andthat the decoder of FIG. 4 is the mirror image of the decoder of FIG. 3.Stated another way, in the case of the encoders, if the four inputsignals are applied to the input terminals of the FIG. 1 encoder in thereverse order from that shown therein (i.e., with the L,, L,, R, and Rsignals applied to input terminals 20', 18', 16 and 14', respectively)the resultant composite signals L and R at output terminals 36 and 40will have the characteristics depicted by phasor groups 66 and 68,respectively (FIG. 2). Conversely, if the four input signals are appliedto the input terminals of the FIG. 2.in reverse order to that shown, thecomposite signals will be as depicted by phasor groups 42 and 44 (FIG.1). Thus, the encoders l and 12 in the system of FIG. may be identicalcircuits with merely the order in which the input signals are appliedand the output signals obtained differing. The encoders being readilyimplementable in integrated circuit form, this feature is ofconsiderable importance in production in that one chip design is neededfor the two different encoding functions.

Likewise, a single circuit, which may also be readily fabricated inintegrated circuit chip form, can serve as either of the two decodersillustrated in FIGS. 3 and 4, simply by the choice of input terminals towhich the two composite signals are applied. That is, if in the circuitof FIG. 3 the input signals L and R are reversed, the output signals atterminals 82, 92, 88 and 84 would be as depicted by phasor groups 126,128, 130 and 132, respectively (FIG. 4). Similarly, reversal of theinput signals to the FIG. 4 matrix would result in decoded signalscorresponding to those illustrated in FIG. 3.

Continuing now the description of the recording system of FIG. 5, thecomposite signals L and R appearing at the output terminals of encoderare amplified by respective amplifiers 150 and 152 and filtered withrespective filter networks 154 and 156, each of which has ahigh-frequency cutoff at a frequency f,, which is at or near the highestaudio frequency of interest, typically 15,000 Hz. The output of filters154 and 156 are respectively passed through suitable delay networks 158and 160, the purpose of which will be described later, and applied tothe left and right input terminals, respectively, ofa stereophoniccutter 162 in cooperative relationship with a disc 164. Thus, the twocomposite signals delivered by matrix 10 are recorded on the two wallsof the disc groove in a manner entirely similar to that used in making aconventional matrixed stereophonic disc as more fully described inapplicants aforementioned application and in his co-pending applicationSer. No. 44,224 filed June 8, 1970, now abandoned. The signals L, and R,cause stylus motion at 90 relative to each other, these motions beingidentical to those found in a conventional stereophonic record. Thequadrature and leading and lagging relationship of the L and R,, signalsin the two composite signals results in the back" channels beingrecorded in the form of clockwise and counter-clockwise helixes. Thus,the right-front and left-front channels have a theoretically infiniteseparation as with a conventional stereophonic record, while theleft-back channel and the right-back channel appear more or lesscentered and somewhat dispersed between the two loudspeakers whenreproduced on a stereophonic player. And, of course, the describedrecording of the back channel information provides the capability ofadmitting to a decoding operation to convert the signals back into adiscrete fourchannel program.

The L and R signals delivered by encoder 12 are recorded in the left andright channels, respectively, as modulation on respective carriersignals. These two composite signals are first amplified by respectiveamplifiers 170 and 172 and then applied to respective modulators 174 and176, each of which may have a self-contained source of carrier signalor, as illustated, they may be energized by an external carrierfrequency generator 178. The function of the modulators 174 and 176, theoperation of which will be described in more detail later, is totranslate the frequency of the signals applied to the encoder 12 to afrequency domain above the baseband spectrum. Typically, the carrierfrequency is of the order of 20 KHz and is amplitude modulated fromabout 5 to 35 KHz. The modulated carriers delivered by the twomodulators are passed through respective filters and 182, each designedto reject the lower sideband (i.e., frequencies below a lower cutofffrequency f leaving essentially only the carrier and the upper sideband.The signals passed by the filters are delayed by respective delaynetworks 184 and 186 and then combined with the output signals fromdelay networks 158 and 160, respectively, for application to the cutter162. The delay networks 158, 160, 184 and 186 are provided to equalizedelays present in the four paths of the system due to the modulationprocess and filtering so that the signals recorded on the disc bear arelative time-domain relationship such that upon replay they can bedecoded and recombined with a minimum of time-delay error.

Referring now to FIG. 5A, which illustrates in more detail a refinementof the modulatiing process shown in FIG. 5, and considering one of themodulators, say modulator 174, after amplification, the L signal isinitially applied to an equalizer circuit 190 which attenuates thecomponents of the composite signal ranging in frequency from about 20 Hzto about 1 KI-Iz to compensate both for the fact that some of the lowersideband signals is transmitted to the output of the processingcircuitry because of the method employed for producing single sideband,and to improve the signal-tonoise ratio of the composite signal uponplayback. The signal-to-noise ratio of the composite signal is improvedby pre-emphasizing the high frequency components of the L signal (inwhich the spectral energy is less than for mid-range frequencies) andcorrespondingly de-emphasizing the signal during playback so that a flatfrequency response is attained and the record noise subsequentlyattenuated. In a preferred embodiment, single sideband is generated bypassing a double sideband signal with carrier through a high-pass filterhaving a cutoff frequency equal to the carrier frequency. Consequently,the audio level for low frequencies must be attenuated with respect tothe high frequencies so that the modulation percentage in the doublesideband region is the same as in the single sideband region forconstant level input. These signals, ranging in frequency from about 17KHz to 20 KHz will be transmitted, albeit with attenuation.

From the equalizer circuit 190 the signal L is applied to an envelopeprocessor citcuit 192 which includes an amplitude modulator 194, a highpass filter 196, a full wave detector 198 and an inverter 200. As willhave become evident, the present invention utilizes a single sidebandmodulator to record the signal L on the same channel (left) of thestereophonic record groove that the signal L is recorded. As isunderstood in the art, when a sideband is removed from an amplitudemodulated carrier signal to develop a single sideband signal withcarrier, the envelope of the single sideband signal is in the shape of apoid and thus represents a distorted replica of the modulating signal.In'

order to compensate for such distortion of the envelope of the singlesideband signal, it is desirable to predistort the modulating signal inthe processor 192.

Within the processor 192, therefore, the signal L is applied to oneinput terminal of the amplitude modulator 194 wherein the signalmodulates a carrier signal having a frequency of the order of 20 KHZ.The developed modulated carrier signal is then applied to the high-passfilter 196 having a cutoff frequency of about 20 KHz which rejects atrapid rate frequencies below 20 KHz and passes without attenuation theupper sideband of the modulated carrier signal. The transmitted sidebandsignal is then applied to the full wave detector circuit 198 whichcomprises, for example, a full wave rectifier and low-pass filter and,accordingly, detects only the envelope of the transmitted sidebandsignal. Thus, the detected signal constitutes a distorted replica of theL signal.

The inverter 200 inverts the detected and distorted L signal and appliesthe inverted signal to one input terminal of an amplitude modulator 202,the first component in single sideband modulator 174. In the modulator174, the inverted and distorted L signal modulates a 20 KHz carriersignal applied from the source 178, for example, to the other inputterninal of the modulator. A high-pass filter 180 having a cutofffrequency f of about 20 KHz transmits only the upper sideband of theamplitide modulated carrier signal. By virtue of the inherent distortionin the modulation process, the envelope of the transmitted uppersideband signal is distorted with the result that the previousdistortion is canceled and the envelope of the resultant signal is anexact replica of the signal L From the filter 180, the upper sidebandsignal with carrier may be amplified by a buffer amplifier 204, theoutput signal from which is applied to the input terminal of delaynetwork 184. It will be understood that the other encoded signal, R isalso processed by a separate like envelope processor and single sidebandmodulator prior to application to delay network 186.

Referring now to HO. 6, there is shown the frequency distribution of theL and L signals, the former being recorded as a regular audio signal andthe latter being recorded as single sideband modulation with carrier Cin the left channel of the record groove. The total bandwidth extendsfrom about 30 Hz to 35 KHz. The distribution of' the frequencycomponents of the right channel signal is identical to that shown forthe left channel. Thus, the resulting disc has a pair of matrix-encodedcomposite signals recorded at baseband, and a pair of carriers singlesideband modulated with differently encoded but related compositesignals, the relationship between the coding of the two sets ofcomposite signals being such that upon replay and decoding, theresulting two sets of four output signals each when combined producefour discrete signals corresponding to the input signals applied to theencoders.

In a preferred embodiment of a four-channel record reproducing systemarranged according to the present invention, as shown in FIG. 7, atwo-channel disc phonograph pickup transducer 210 includes a stylus 212that responds to the modulations in the groove of a record disc recordedin the above-described manner to supply the L signal and the L singlesideband signal with carrier, and the R signal together with the Rsingle sideband signal with carrier, along a pair of conductors 214 and216, respectively, to a pair of preamplifiers 218 and 220, respectively.The stereophonic pickup has a good frequency response up to the highestfrequency of interest of the modulated carrier recorded on therecord,namely, up to about 35 KHz. The output signal from preamplifier218 is applied in parallel to a low-pass filter 222 designed to transmitaudio frequencies up to a frequency f which is at or near the upperfrequency of the baseband signals being reproduced, and to a high-passfilter 224 designed to transmit frequencies above the frequency f which,typically, includes a part of the carrier and all of the upper sidebandof the modulated signal. The signal from preamplifier 220 is likewiseapplied in parallel to a low-pass filter 226 and a high-pass filter 228,each having characteristics corresponding to the characteristics ofjustdescribed filters 222 and 224, respectively.

The signals transmitted by low-pass filters 222 and 226, namely, L and Rare applied to the input terminals and 72, respectively, of a decoder230, having the configuration illustrated in FlG. 3, which decodes thecomposite signals in the manner described earlier to produce at the fouroutput terminals thereof the signals L' L' R' and R',, containing thesignal components depicted by phasor groups 94, 96, 98 and 100,respectively (FIG. 3).

The output signals from high-pass filters 224 and 228 are applied torespective detectors 232 and 234 which are operative to detect themodulation on the carriers and recover the original encoded compositesignals L and R respectively. The output signals L and R from thedetectors are applied to input terminals 102 and 104, respectively, of asecond decoder 236, having the configuration illustrated in FIG. 4,which is operative to decode the composite signals in the manner earlierdescribed to produce at its four output terminals the signals L' L' Rand R',, containing the signal components depicted by phasor groups and132, respectively (FIG. 4).

The signals at corresponding output terminals of decoders 230 and 236are then summed at respective summing junctions 238, 240, 242 and 244,in the manner described earlier in connection with FIGS. 3 and 4, toyield at the output conductors 246, 248, 250 and 252 four discretesignals L,, L, R and R,, respectively. These four signals may then beapplied, with or without further amplification, to four separateloudspeakers (not shown) positioned at the left front, left back, rightback and right front corners, respectively, of a listening area toreproduce sounds corresponding in every way to the original signalsapplied to the encoders to provide completely independent transmissionof the four channels.

It will have become evident that a disc record recorded in accordancewith the invention, and the reproducing apparatus illustrated in FIG. 7,offer the important advantage that the record can be satisfactorilyreproduced with other existing types of playback equipment, and that thereproducing apparatus is capable of reproducing existing recordsrecorded in other modes. Considering the record first, as has beenmentioned earlier, it can be replayed on a conventional stereophonicphonograph because of the presence in the encoded baseband signals ofall left and right front signals so encoded as to provide a satisfactorystereophonic presentation on two loud-speakers. The record can also bereplayed on existing stereo-quadraphonic playback apparatus of the kindillustrated in aforementioned application Ser. No. 124,135 whichincludes a single matrix decoder having the construction illustrated inFIG. 3 and capable of decoding the composite baseband signals to derivefour output signals in which the L,, L,,, R, and R, signals arerespectively predominant. It follows that the record can also bereproduced on a stereo-quadraphonic logic decoder of the kind alsoillustrated in application Ser. No. 124,135 which includes in additionto a matrix decoder, separate gain control amplifiers for the fouroutput signals from the decoder and a logic and control circuitoperative in response to the signals instantaneously present to adjustthe gains of the amplifiers to enhance channel separation and thus therealism of four channel reproduction.

Conversely, a conventional stereophonic record will be satisfactorilyreproduced on the playback apparatus of FIG. 7; in this case the leftand right signals are transduced and simply passed by low-pass filters222 and 226 to the input terminals of decoder 230 which, because of thedirect connections (except for the reference phase shift) of the inputterminals to the left-front and right-front output terminals, willdeliver them to the appropriate loudspeakers. Likewise, existingstereo-quadraphonic records recorded in accordance with the teaching ofapplication Ser. No. 124,135 (i.e., using an encoder of the kind shownin FIG. 1), and being distributed by Columbia Records, when replayed onthe apparatus of FIG. 7 will produce at the output terminals of decoder230 four output signals in which Ly, L R, and R signals are respectivelypredominant.

Although not required for playback of records recorded in accordancewith the present invention, the capability of the playback apparatus ofFIG. 7 to reproduce existing stero-quadraphonic records may be extendedby further including gain control amplifiers 260, 262, 264, and 266 inthe output lines 246, 248, 250 and 252, respectively, and logic andcontrol circuitry 268 of the kind shown in application Ser. No. 124,135for example, for adjusting the gains of the amplifiers to enhancechannel separation. The logic and control circuit is operative inresponse to signals derived from decoder 230 to deliver appropriatecontrol signals to the control electrodes of the gain controlamplifiers.

Inasmuch as the just-described logic control is desired only whenreplaying existing stereo-quadraphonic records on which the encodedsignals are recorded at baseband, the system of FIG. 7 further includesmeans fpr automatically disabling the logic circuit 268 when a recordaccording to the present invention is being played and for activatingthe logic when the existing type of stereo-quadraphonic record is beingplayed. To this end, the output terminals of high pass filters 224 and228 are connected to the input terminals of respective isolatingamplifiers 270 and 272, the output signals from which are rectified inrespective rectifiers 274 and 276. The signals delivered by therectifiers are summed in a summing or combining junction 278, the outputof which is connected through the actuating coil 280 of a relay 282. Thecontact 284 of the relay is connected in series with suitable circuitrywithin logic and control circuit 268 which is operative when the contactis closed to actuate the logic. Thus, if an existing type ofstereo-quadraphonic record is being played, the absence of anysignificant carrier signal output from filters 224 and 228 will causethe relay contact to remain closed and the logic to function in themanner described earlier.

However, when a record according to the present invention is played, theamplified and rectified carrier signals passed by filters 224 and 228cause sufficient current to flow in relay coil 280 to open the contact284 and disable the logic to prevent application of variable controlsignals to the gain control amplifiers 260 and 266 and instead establisha constant bias on the amplifiers so that they function as fixed gainamplifiers. In this case, then, the two decoders operate in the mannerdescribed earlier to produce discrete or independent output channels L,,L,,, R and R, at the output terminals of the respective amplifiers.

It will be evident to ones skilled in the art that the describedcarrier-actuated relay may be refined by the use of tuned circuits toavoid operation of the relay merely by surface noise or extraneoussignals that may be present in the record. Also, it is advisable todesign the rectifiers 274 and 276 to have attack and release timeconstants to preclude actuation of the relay by spurious pulses that maybe found on the record. It will be evident, too, that the describedcarrier-actuated switch may also be used to adjust the gain ofamplifiers 260, 262, 264 and 266 in a predetermined manner so as toaccomodate the level of response of the system to the needs of thevarious types of records that may be played.

Although the invention has been described as utilizing combinations ofencoders and decoders of specific design, it is possible to modify theencoders, for example, without changing the decoders, and vice versa,and still realize the benefits of the invention. For example, byswitching the switches S and S in the encoders of FIGS. 1 and 2 to theother contact from that shown, the full left-front and right-frontsignals are caused to bypass their respective dJ-networks and areapplied directly to an input terminal of summing junction 34 and 38(FIG. 1) or 58 and 62 (FIG. 2), respectively. The L,, and R, signals areapplied (in the case of the FIG. 1 encoder) to summing networks 22 and26, ill-networks 30 and 32 and summing junctions 34 and 38, as before.

The effect of this modification is illustrated in the phasor diagrams 42and 44 associated with FIG. 1 and in phasor diagrams 64 and 66associated with FIG. 2. Considering the phasor diagrams 42 and 44, it isseen that when the L, and R, signals by-pass their respectiveill-networks 24 and 28, these phasors L, and R,in effect are moved in aleading direction by the phase angle #:(f), which for a given frequencymay introduce the illustrated phase shift ill". Since the ill-functionsin all of the ip-networks are substantially the same, the resultingphasors L, and R, in phasor groups 66 and 68, therefore remain in thesame relative phase relationship with respect to each other, but areshifted relative to the phasors 0.707R and 0.707L,, in said phasorgroups, the latter two sets of phasors, however, being unaffected.

This seemingly minor modification provides significant improvement incertain recording environments. For example, a center total signalapplied equally to all of the input terminals l4, l6, l8 and 20 of theencoder of FIG. 1 with the switches in the positions illustrated wouldif applied directly to a four-channel reproduction system, appear as anoverhead signal; thus, a corresponding signal recorded for two-channelstereophonic reproduction should appear as a generally center signal inthe stereophonic field. However, with the unmodified FIG. 1 encoder itcan be shown that a center total" signal at input terminal 14 ischaracterized by the addition of the three fundamental phasors LO.707R,,, and 0.707L which results in a relatively large center totalsignal in the left" channel. Similarly, at the output terminal 40 thecenter total signal is made up of the phasor sum of the three basicsignals R 0.707R and 0.707L,,, which is smaller than that appearing atoutput terminal 36. Thus under the circumstances mentioned above,instead of obtaining two signals of generally equal magnitudes, theencoder of FIG. 1 (unmodified) produces two composite signals ofgenerally unequal magnitudes. By-passing the ill-networks 24 and 28 (byswitching switches S and S to the other position) in essence introducesa differential reference function tb" between the front signals L, andR; and the rear signals R and L Therefore, the phasors corresponding tothese sets of signals are positioned differently with respect to eachother at different frequencies, resulting in an action similar to thatof a comb filter in equalizing the magnitudes of the phasors resultingfrom application of a center total signal equally to all four inputterminals of the encoder. 7

Referring now to the phasor diagrams in FIGS. 3 and 4, it is seen thatif all of the phasors L,, R,, 0.707L, and 0.707R, therein are shifted inphase by the same angle 11:", as shown in FIGS. 1 and 2, their relativeposition in the phasor groups will be changed with respect to thephasors L,,, R,,, 0.707L and 0.707R by the same phase angle ill", butsince all of the phasors will have been shifted simultaneously (as shownby the dashed line phasors) by the same angle ill" (omitted for clarityin FIGS. 3 and 4), the positions of the phasors L,, R,, 0.707L, and0.707R, are relatively unchanged. Because of this relatively unchangedposition of these phasors, when the two sets of four output signalsappearing at output terminals of the FIG. 3 and FIG. 4 decoders aresummed together in the manner shown in FIG. 7, the transferred signalcomponents 0.707 L, and 0.707L will still cancel each other. Therefore,the illustrated modification of the encoder will not interfere with theoperation of the overall system. The essential criteria is that thecoding sequences of the two pairs of encoded signals be complementary soas to permit positioning for cancellation of the unwanted signals whenthe two sets of decoded signals are added together or otherwise respondto the theory described in connection with equations (1 )-(8).

Although the recording and playback apparatus described above utilizethe single sideband modulation principle, other types of modulation canbe used to modulate the carrier. For example, it is possible to useconventional (double sideband) amplitude modulation, in which thecarrier would be placed at about 35 KHz with the sidebands extendingfrom to 50 KHz. Also, frequency or phase methods of modulation, or otherforms of modulation, could be used without departing from the spirit ofthe invention.

it should also be noted that the switchable encoder arrangement shown inFIG. l,for example, may be used to record stereo-quadraphonic records ofthe existing type (non-carrier) in the manner described inaforementioned application Ser. No. 124,135.

Furthermore, the method and apparatus disclosed herein is equallyapplicable to the transmission of discrete independent channels oversuitable broadcasting facilities, while retaining all the benefits ofthe compatible quadraphonic matrix system herein described. Theimplementation of this method is carried out, for example, by encodingthe four channels with the encoder of FIG. 1 for transmission over thebroadcasting channels which are received by conventional stereophonicreception systems, e.g., an F.M.-multiplex receiver, and by encodingagain the four channels using the encoder of FIG. 2 for transmission ontwo additional channels or sub-carriers which are received by anauxiliary receiver circuit. Upon detection and rematrixing in a mannersimilar to that shown in FIG. 7 four discrete output signals areproduced. Other modifications of this alternative application of theinvention will be evi dent to those skilled in the art.

I claim: 1. A sound system for transmitting four input audio informationsignals designated L;, L, R,, and R, on a transmission medium havingfirst and second channels each of which has a baseband frequency channeland a modulated carrier channel and for reproducing correspondingsubstantially identical audio information output signals, comprising,incombination, an encoder and a decoder, said encoder comprising meansfor coupling to said first channel of said medium a first combinedsignal containing a first composite signal at baseband frequencycontaining said L L and R,, signals to the extent they are present and acarrier signal modulated by a second composite signal containing saidL,, L and R signals, the L and R, signals in said first composite signalbeing in phase opposition with corresponding sig nals in said secondcomposite signal, and means for coupling to said second channel of saidmedium a second combined signal containing a third composite signal atsaid baseband frequency containing said R L, and R signals to the extentthey are present and a carrier signal modulated by a fourth compositesignal containing said R L and R signals, the L,, and R signals in saidthird composite signal being in phase opposition with the correspondingsignals in said fourth composite signal, and said decoder comprisingfilter means for separating said first and said third composite signalsat baseband frequency from the modulated carrier signals in each of saidfirst and second combined signals,

first and second detector means for respectively detecting themodulation from the modulated carrier signals from said first and secondcombined signals and operative to derive said second and fourthcomposite signals, respectively,

first decoding means to which said separated first and third compositesignals are applied,

second decoding means to which said derived second and fourth compositesignals are applied,

said first and second decoding means each being operative in response tothe signals applied thereto to derive four output signals respectivelycontaining dominant signal components L R L,, and R each accompanied bysub-dominant signals, and

means for combining the four output signals from said first decodingmeans with the corresponding four output signals from said seconddecoding means and operative to cancel said sub-dominant signals and toproduce four discrete output signals L], L R and R 2. A sound systemaccording to claim 1 wherein said L, and R signals in said first andsecond composite signals are substantially in quadrature with the L andR signals in said third and fourth composite signals, respectively.

3. A sound system according to claim 2 wherein the amplitudes of said L,and R signals in said composite signals are reduced by a factor ofapproximately 0.707 relative to the amplitudes of said L, and R,signals.

4. A sound system according to claim 3 wherein said L, and R, signals insaid composite signals are substantially in phase with each other.

5. A sound system according to claim 3 wherein at least one of said Land R signals is either in phase or in phase opposition with thedominant signal in each of said first, second, third and fourthcomposite signals.

6. For use with a sound system wherein four discrete audio informationsignals designated L,, L,,, R, and R, to the extent they are present arerecorded or transmitted on first and second channels of a two-channelmedium each channel of which has a baseband frequency channel and amodulated carrier channel as first and second combined signals,respectively, the first combined signal containing a first compositesignal at baseband frequency containing said L L and R signals and acarrier signal modulated by a second composite signal containing saidL,, L, and R, signals, the L and R signals in said first compositesignal being in phase opposition with corresponding signals in saidsecond composite signal, and the second combined signal containing athird composite signal at baseband frequency containing said R,, L, andR,, signals and a carrier signal modulated by a fourth composite signalcontaining said R,, L,, and R signals, the L, and R signals in saidthird composite signal being in phase opposition with the correspondingsignals in said fourth composite signals, a decoder for recovering saidfour audio information signals comprising, in combination,

filter means for separating said first and third composite signals atbaseband frequency from the modulated carrier signals in each of saidfirst and second combined signals,

first and second detector means for respectively detecting themodulation from the modulated carrier signals from said first and secondcombined signals to derive said second and fourth composite signals,respectively,

first decoding means to which said separated first and third compositesignals are applied,

second decoding means to which said derived second and fourth compositesignals are applied, said first and second decoding means each beingoperative in response to the signals applied thereto to derive fouroutput signals respectively containing dominant signal componentsl R;,L, and R each accompanied by sub-dominant signals, and

combining means for combining the four output signals from said firstdecoding means with the corresponding four output signals from saidsecond decoding means and operative to cancel said subdominant signalsand to produce four discrete output signals L,, L,,, R and R 7.Apparatus in accordance with claim 6 wherein said filter means comprisesfirst and second low-pass filters each having a cutoff frequency atapproximately the highest baseband frequency for respectivelytransmitting said first and third composite signals at basebandfrequency, and

for respectively transmitting the modulated carrier signals in saidfirst and second combined signals.

8. Apparatus according to claim 7 wherein said carrier signals areamplitude modulated and said first and second detector means areamplitude modulation detectors respectively connected to receive thesignals transmitted by said first and second high-pass filters.

9. Apparatus in accordance with claim 8 further including first, second,third and fourth gain control means to which the four output signalsfrom said combining means are respectively applied,

control circuit means operative in response to said separated first andthird composite signals to selectively control the gains of said gaincontrol means, and

means operative in response to the presence of carrier signals at theoutputs of said first and second high-pass filters to disable saidcontrol circuit, 10. A method for recording four audio information inputsignals on a two-channel stereophonic disc record, and for reproducingsaid four audio information signals as discrete signals, comprising,

encoding said four input signals by combining them in preselectedamplitude and phase relationships to form first and second compositesignals at baseband frequency each including at least three of saidinput signals, encoding said four input signals by combining them inpreselected amplitude and phase relationships to form third and fourthcomposite signals said third and fourth composite signals respectivelyincluding the same at least three of said input signals as are includedin said first and second composite signal,

modulating first and second carrier frequency signals with said thirdand fourth composite signals, respectively,

recording said first composite signal at baseband frequency and saidfirst modulated carrier signal on one channel of a stereophonic discrecord and recording said second composite signal at baseband frequencyand said second modulated carrier signal on the other channel of saidstereophonic disc record;

transducing from the stereophonic disc record first and second combinedsignals recorded on the respective channels thereof,

separating said first and third composite signals at baseband frequencyfrom the modulated carrier signals in each of said first and secondcombined signals,

detecting the modulation from the separated modulated carrier signals toderive said second and fourth composite signals, respectively,

decoding said separated first and third composite signals to produce afirst set of four composite output signals containing dominant signalcomponents corresponding to respective ones of said four input signalseach accompanied by sub-dominant signals, decoding said derived secondand fourth output signals to produce a second set of four compositeoutput signals containing dominant signal components corresponding torespective ones of said four input signals each accompanied by the samesubdominant signals as accompany corresponding ones of the four outputsignals resulting from decoding said first and third composite signalsbut in phase relationships such that the sub-dominant signal componentscontained in the four output signals of the first set are in phaseopposition with corresponding sub-dominant signal components containedin corresponding ones of the four output signals of the second set, andcombining the four output signals of said first set with thecorresponding four output signals of said second set for canceling saidsub-dominant signals and producing four discrete output signalscorresponding to respective ones of said four input signals.

1 1. In a sound system for transmitting a multichannel program includingfirst, second, third and fourth program signals to the extent they arepresent to respective separate loudspeakers on a transmission mediumhaving first and second channels each of which has a baseband frequencychannel and a modulated carrier channel, the combination comprising:

first and second matrix encoders each having first,

second, third and fourth input terminals to which said first, second,third and fourth signals are respectively applied and first and secondoutput terminals, each said encoder including means connected betweensaid first and fourth input terminals and said first and second outputterminals, respectively, operative to transfer substantially equalamplitude proportions of said first and fourth signals to said first andsecond output terminals, respectively, and

means connected between both said second and third input terminals andboth said first and second output terminals operative to transfersubstantially equal reduced amplitude proportions of said second andthird signals to both of said first and second output terminals andincluding phase-shifting networks operative to provide a substantiallyconstant differential phase-shift angle between said second and thirdsignals at said output terminals over the frequency range of interest;

first means for coupling to the first channel of said transmissionmedium a first combined signal containing a first composite signal fromthe first output terminal of said first matrix encoder at basebandfrequency and a carrier signal modulated by a second composite signalfrom the first output terminal of said second matrix encoder; and

second means for coupling to the second channel of said transmissionmedium a second combined signal containing a third composite signal fromthe second output terminal of said first matrix encoder at basebandfrequency and a carrier signal modulated by a fourth composite signalfrom the second output terminal of said second matrix encoder.

12. Apparatus according to claim 11 wherein said differentialphase-shift angle has a value of substantially 90 and said second signalat one of the output terminals of each of said matrix encoders leadssaid second signal at the other output terminal of the respective matrixencoder and said third signal at said one output terminal of eachencoder lags said. third signal at the other output terminal of therespective matrix encoder.

13. Apparatus in accordance with claim 12, wherein said second and thirdsignals in said first and third composite signals are substantially inphase opposition with the second and third signals in said second andfourth composite signals, respectively.

14. Apparatus in accordance with claim 13, wherein said first and secondsignals in said first, second, third and fourth composite signals aresubstantially in phase with each other.

15. Apparatus in accordance with claim 14,,wherein the amplitudes ofsaid second and third signals in said composite signals are reduced by afactor of substantially 0.707 relative to the amplitudes of said firstand second signals.

16. in a sound system for transmitting or recording four discrete audioinformation signals designated L,, L,,, R, and R, on a transmission orrecording medium having first and second channels each of which has abaseband frequency channel and a modulated carrier channel wherein firstand second combined signals containing predetermined matrixedcombinations of said four audio information signals are applied to thefirstand second channels of said medium, respectively, and includingmeans for deriving from said first and second combined signals first andsecond sets of four output signals, corresponding ones in each setcontaining dominant signal components L,, R,, L, and R each accompaniedby sub-dominant signals, and means for combining the four output signalsof the first set with the corresponding four output signals of thesecond set to produce four discrete output signals L,, L, R,, and Rtransmitting or recording apparatus, comprising:

means including matrix encoding means for coupling to said first channelof said transmission or recording medium a first combined signalcontaining a first composite signal at said baseband frequencycontaining said L;, L, and R signals to the extent they are present anda carrier signal modulated by a second composite signal containing saidL L and R, signals, the L, and R, signals in said first composite signalbeing in phase opposition with the L and R signals in said secondcomposite signal, and

means including said matrix encoding means for coupling to said secondchannel of said transmission or recording medium a second combinedsignal con taining a third composite signal at said baseband frequencycontaining said R,, L, and 'R signals and a carrier signal modulated bya fourth composite signal containing said R,, L, and R signals, the L,and R signals in said third composite signal being in phase oppositionwith the L, and R,, signals in said fourth composite signal.

17. Apparatus according to claim 16, wherein said L, and R signals insaid first and second composite signals are substantially in quadraturewith the L and R signals in said third and fourth composite signals,respectively, and the L and R signals in said first and third compositesignals are substantially in phase opposition with the L, and R, signalsin said second and fourth composite signals, respectively.

' 18. Apparatus according to claim 17, wherein said L, and R, signals insaid first, second, third and fourth composite signals are substantiallyin phase with each other.

19. Apparatus in accordance with claim 18, wherein the amplitudes ofsaid L, and R signals in said first, second, third and fourth compositesignals are reduced by a factor of substantially 0.707 relative to theamplitudes of said L, and R, signals.

l i i i i

1. A sound system for transmitting four input audio information signalsdesignated Lf, Lb, Rb and Rf on a transmission medium having first andsecond channels each of which has a baseband frequency channel and amodulated carrier channel and for reproducing correspondingsubstantially identical audio information output signals, comprising, incombination, an encoder and a decoder, said encoder comprising means forcoupling to said first channel of said medium a first combined signalcontaining a first composite signal at baseband frequency containingsaid Lf, Lb and Rb signals to the extent they are present and a carriersignal modulated by a second composite signal containing said Lf, Lb andRb signals, the Lb and Rb signals in said first composite signal beingin phase opposition with corresponding signals in said second compositesignal, and means for coupling to said second channel of said medium asecond combined signal containing a third composite signal at saidbaseband frequency containing said Rf, Lb and Rb signals to the extentthey are present and a carrier signal modulated by a fourth compositesignal containing said Rf, Lb and Rb signals, the Lb and Rb signals insaid third composite signal being in phase opposition with thecorresponding signals in said fourth composite signal, and said decodercomprising filter means for separating said first and said thirdcomposite signals at baseband frequency from the modulated carriersignals in each of said first and second combined signals, first andsecond detector means for respectively detecting the modulation from themodulated carrier signals from said first and second combined signalsand operative to derive said second and fourth composite signals,respectively, first decoding means to which said separated first andthird composite signals are applied, second decoding means to which saidderived second and fourth composite signals are applied, said first andsecond decoding means each being operative in response to the signalsapplied thereto to derive four output signals respectively containingdominant signal components Lf, Rf, Lb and Rb each accompanied bysub-dominant signals, and means for combining the four output signalsfrom said first decoding means with the corresponding four outputsignals from said second decoding means and operative to cancel saidsubdominant signals and to produce four discrete output signals Lf, Lb,Rb and Rf.
 2. A sound system according to claim 1 wherein said Lb and Rbsignals in said first and second composite signals are substantially inquadrature with the Lb and Rb signals in said third and fourth compositesignals, respectively.
 3. A sound system according to claim 2 whereinthe amplitudes of said Lb and Rb signals in said composite signals arereduced by a factor of approximately 0.707 relative to the amplitudes ofsaid Lf and Rf signals.
 4. A sound system according to claim 3 whereinsaid Lf and Rf signals in said composite signals are substantially inphase with each other.
 5. A sound system according to claim 3 wherein atleast one of said Lb and Rb signals is either in phase or in phaseopposition with the dominant signal in each of said first, second, thirdand fourth composite signals.
 6. For use with a sound system whereinfour discrete audio information signals designated Lf, Lb, Rb and Rf tothe extent they are present are recorded or transmitted on first andsecond channels of a two-channel medium each channel of which has abaseband frequency channel and a modulated carrier channel as first andsecond combined signals, respectively, the first combined signalcontaining a first composite signal at baseband frequency containingsaid Lf, Lb and Rb signals and a carrier signal modulated by a secondcomposite signal containing said Lf, Lb and Rb signals, the Lb and Rbsignals in said first composite signal being in phase opposition withcorresponding signals in said second composite signal, and the secondcombined signal containing a third composite signal at basebandfrequency containing said Rf, Lb and Rb signals and a carrier signalmodulated by a fourth composite signal containing said Rf, Lb and Rbsignals, the Lb and Rb signals in said third composite signal being inphase opposition with the corresponding signals in said fourth compositesignals, a decoder for recovering said four audio information signalscomprising, in combination, filter means for separating said first andthird composite signals at baseband frequency from the modulated carriersignals in each of said first and second combined signals, first andsecond detector means for respectively detecting the modulation from themodulated carrier signals from said first and second combined signals toderive said second and fourth composite signals, respectively, firstdecoding means to which said separated first and third composite signalsare applied, second decoding means to which said derived second andfourth composite signals are applied, said first and second decodingmeans each being operative in response to the signals applied thereto toderive four output signals respectively containing dominant signalcomponents Lf, Rf, Lb and Rb each accompanied by sub-dominant signals,and combining means for combining the four output signals from saidfirst decoding means with the corresponding four output signals fromsaid second decoding means and operative to cancel said sub-dominantsignals and to produce four discrete output signals Lf, Lb, Rb and Rf.7. Apparatus in accordance with claim 6 wherein said filter meanscomprises first and second low-pass filters each having a cutofffrequency at approximately the highest baseband frequency forrespectively transmitting said first and third composite signals atbaseband frequency, and first and second high-pass filters each having acutoff frequency at approximately the carrier frequency for respectivelytransmitting the modulated carrier signals in said first and secondcombined signals.
 8. Apparatus according to claim 7 wherein said carriersignals are amplitude modulated and said first and second detector meansare amplitude modulation detectors respectively connected to receive thesignals transmitted by said first and second high-pass filters. 9.Apparatus in accordance with claim 8 further including first, second,third and fourth gain control means to which the four output signalsfrom said combining means are respectively applied, control circuitmeans operative in response to said separated first and third compositesignals to selectively control the gains of said gain control means, andmeans operative in response to the presence of carrier signals at theoutputs of said first and second high-pass filters to disable saidcontrol circuit.
 10. A method for recording four audio information inputsignals on a two-channel stereophonic disc record, and for reproducingsaid four audio information signals as discrete signals, comprising,encoding said four input signals by combining them in preselectedamplitude and phase relaTionships to form first and second compositesignals at baseband frequency each including at least three of saidinput signals, encoding said four input signals by combining them inpreselected amplitude and phase relationships to form third and fourthcomposite signals said third and fourth composite signals respectivelyincluding the same at least three of said input signals as are includedin said first and second composite signal, modulating first and secondcarrier frequency signals with said third and fourth composite signals,respectively, recording said first composite signal at basebandfrequency and said first modulated carrier signal on one channel of astereophonic disc record and recording said second composite signal atbaseband frequency and said second modulated carrier signal on the otherchannel of said stereophonic disc record; transducing from thestereophonic disc record first and second combined signals recorded onthe respective channels thereof, separating said first and thirdcomposite signals at baseband frequency from the modulated carriersignals in each of said first and second combined signals, detecting themodulation from the separated modulated carrier signals to derive saidsecond and fourth composite signals, respectively, decoding saidseparated first and third composite signals to produce a first set offour composite output signals containing dominant signal componentscorresponding to respective ones of said four input signals eachaccompanied by sub-dominant signals, decoding said derived second andfourth output signals to produce a second set of four composite outputsignals containing dominant signal components corresponding torespective ones of said four input signals each accompanied by the samesub-dominant signals as accompany corresponding ones of the four outputsignals resulting from decoding said first and third composite signalsbut in phase relationships such that the sub-dominant signal componentscontained in the four output signals of the first set are in phaseopposition with corresponding sub-dominant signal components containedin corresponding ones of the four output signals of the second set, andcombining the four output signals of said first set with thecorresponding four output signals of said second set for canceling saidsub-dominant signals and producing four discrete output signalscorresponding to respective ones of said four input signals.
 11. In asound system for transmitting a multichannel program including first,second, third and fourth program signals to the extent they are presentto respective separate loudspeakers on a transmission medium havingfirst and second channels each of which has a baseband frequency channeland a modulated carrier channel, the combination comprising: first andsecond matrix encoders each having first, second, third and fourth inputterminals to which said first, second, third and fourth signals arerespectively applied and first and second output terminals, each saidencoder including means connected between said first and fourth inputterminals and said first and second output terminals, respectively,operative to transfer substantially equal amplitude proportions of saidfirst and fourth signals to said first and second output terminals,respectively, and means connected between both said second and thirdinput terminals and both said first and second output terminalsoperative to transfer substantially equal reduced amplitude proportionsof said second and third signals to both of said first and second outputterminals and including phase-shifting networks operative to provide asubstantially constant differential phase-shift angle between saidsecond and third signals at said output terminals over the frequencyrange of interest; first means for coupling to the first channel of saidtransmission medium a first combined signal containing a first compositesignal from the first output terminal of saiD first matrix encoder atbaseband frequency and a carrier signal modulated by a second compositesignal from the first output terminal of said second matrix encoder; andsecond means for coupling to the second channel of said transmissionmedium a second combined signal containing a third composite signal fromthe second output terminal of said first matrix encoder at basebandfrequency and a carrier signal modulated by a fourth composite signalfrom the second output terminal of said second matrix encoder. 12.Apparatus according to claim 11 wherein said differential phase-shiftangle has a value of substantially 90* and said second signal at one ofthe output terminals of each of said matrix encoders leads said secondsignal at the other output terminal of the respective matrix encoder andsaid third signal at said one output terminal of each encoder lags saidthird signal at the other output terminal of the respective matrixencoder.
 13. Apparatus in accordance with claim 12, wherein said secondand third signals in said first and third composite signals aresubstantially in phase opposition with the second and third signals insaid second and fourth composite signals, respectively.
 14. Apparatus inaccordance with claim 13, wherein said first and second signals in saidfirst, second, third and fourth composite signals are substantially inphase with each other.
 15. Apparatus in accordance with claim 14,wherein the amplitudes of said second and third signals in saidcomposite signals are reduced by a factor of substantially 0.707relative to the amplitudes of said first and second signals.
 16. In asound system for transmitting or recording four discrete audioinformation signals designated Lf, Lb, Rb and Rf on a transmission orrecording medium having first and second channels each of which has abaseband frequency channel and a modulated carrier channel wherein firstand second combined signals containing predetermined matrixedcombinations of said four audio information signals are applied to thefirst and second channels of said medium, respectively, and includingmeans for deriving from said first and second combined signals first andsecond sets of four output signals, corresponding ones in each setcontaining dominant signal components Lf, Rf, Lb and Rb each accompaniedby sub-dominant signals, and means for combining the four output signalsof the first set with the corresponding four output signals of thesecond set to produce four discrete output signals Lf, Lb, Rb and Rf,transmitting or recording apparatus, comprising: means including matrixencoding means for coupling to said first channel of said transmissionor recording medium a first combined signal containing a first compositesignal at said baseband frequency containing said Lf, Lb and Rb signalsto the extent they are present and a carrier signal modulated by asecond composite signal containing said Lf, Lb and Rb signals, the Lband Rb signals in said first composite signal being in phase oppositionwith the Lb and Rb signals in said second composite signal, and meansincluding said matrix encoding means for coupling to said second channelof said transmission or recording medium a second combined signalcontaining a third composite signal at said baseband frequencycontaining said Rf, Lb and Rb signals and a carrier signal modulated bya fourth composite signal containing said Rf, Lb and Rb signals, the Lband Rb signals in said third composite signal being in phase oppositionwith the Lb and Rb signals in said fourth composite signal. 17.Apparatus according to claim 16, wherein said Lb and Rb signals in saidfirst and second composite signals are Substantially in quadrature withthe Lb and Rb signals in said third and fourth composite signals,respectively, and the Lb and Rb signals in said first and thirdcomposite signals are substantially in phase opposition with the Lb andRb signals in said second and fourth composite signals, respectively.18. Apparatus according to claim 17, wherein said Lf and Rf signals insaid first, second, third and fourth composite signals are substantiallyin phase with each other.
 19. Apparatus in accordance with claim 18,wherein the amplitudes of said Lb and Rb signals in said first, second,third and fourth composite signals are reduced by a factor ofsubstantially 0.707 relative to the amplitudes of said Lf and Rfsignals.